VoIP Over IEEE 802.11 Wireless Networks: Experimental Analysis of Interference Effects Leopoldo Angrisani (1) , Aniello Napolitano (2) and Alessandro Sona (3) (1) Dept. of Computer Science and Control Systems, Univ. of Naples Federico II, via Claudio 21, 80125, Naples, Italy Phone: +39 081 7683170, Fax: +39 081 7683816, email: angrisan@unina.it (2) Dept. of Electrical Engineering, Univ. of Naples Federico II, via Claudio 21, 80125, Naples, Italy Phone: +39 081 7683866, Fax: +39 081 2396897, email: aniello.napolitano@unina.it (3) Dept. of Information Engineering – Univ. of Padova, via G. Gradenigo, 6 – 35131 Padova, Italy Phone: +39 0498277743, Fax: +39 0498277699, email: alessandro.sona@unipd.it Abstract— The interference effects on Voice over Internet Protocol (VoIP) applications over Wireless Local Area Networks (WLANs) are here dealt with. In particular, attention is paid to the IEEE 802.11g standard, with Bluetooth signals, Additive White Gaussian Noise (AWGN) and competitive data traffic acting as interferer. The analysis is conducted experimentally following a cross-layer approach and through the deployment of a proper testbed within a semi-anechoic chamber. The purpose is to investigate on the effects of some common types of interference on a WLAN supporting VoIP applications, and to deduce from the experiments valuable hints for improving the setup of a WLAN in the presence of interference. Cross-layer measurements are performed at network/transport, physical and application layer. Keywords: Radiofrequency (RF) measurements, channel power, signal-to-interference power ratio (SIR), packet loss ratio (PLR), VoIP over Wi-Fi, wireless networks. I. INTRODUCTION Voice over Wireless Local Area Network (VoWLAN) is an emerging internet technology [1], integrating the Voice over Internet Protocol (VoIP) [2] over Wireless Local Area Networks (WLANs). VoWLAN is designed to provide efficient telephony applications alternative to cellular telephone networks. It integrates wired and wireless telephony in the same Internet Protocol (IP) infrastructure, reducing calling charges, and avoiding the typical problems of the highly variable coverage of the cell phone networks inside buildings. One first fundamental problem of VoWLAN is that VoIP traffic and data traffic from traditional applications such as web and e-mail, can interfere with each other, leading to reduced performance. A second issue is the poor stability and reliability of the deployed radio links. In fact, VoWLAN is based on the IEEE 802.11 standard [3] (informally known collectively as Wi-Fi), which employs scarce, shared, and noisy spectrum, namely the unlicensed 2.4 GHz Industrial Scientific Medical (ISM) band. In these bands other devices may operate simultaneously, like IEEE 802.15.4 (Zig-Bee) [4] and IEEE 802.16 (Bluetooth) [5] apparatuses, microwave ovens, cordless phones, baby monitors, security cameras, adding significant interference and provoking data packet collisions. Because of such interference phenomena, even strong reduced performance in terms of speech quality can be perceived by end users. To properly face the problem, the radio link characteristics should be carefully known and accounted for just at the early stages of a WLAN design and setup. But, such information are not easy-to-obtain and often require to perform ad-hoc laboratory and on- the-field measurements. In such a direction, useful information can be achieved through cross-layer measurements, through which the origin of some issues and drawbacks can efficiently be detected and solved [6]. A number of previous works on VoIP over WLAN and related interference phenomena are available in the literature [7-14]. In these papers, various problems are investigated and solutions are proposed aimed at improving the capability of VoIP applications and system reliability. For instance, in [8] the coexistence of Transmission Control Protocol (TCP) and VoIP traffic in WLAN is studied in terms of delays and loss of performance. In [12], experimental studies are shown on the throughput of IEEE 802.11b wireless networks for user datagram protocol (UDP) and VoIP traffic. In all these contributions, attention is mainly paid to only interference at network/transport layer, due to the presence of data traffic on the same WLAN. Very few information and results are instead available in terms of physical layer interference. In this paper, the use of a IEEE 802.11g WLAN to provide VoIP services is analyzed through a cross-layer measurement approach [6]. The goal is twofold: first to experimentally investigate on the interference effects on a WLAN supporting VoIP services, at both network/transport and physical layer; second, to deduce hints for improving the setup of a WLAN transporting 978-1-4244-2737-6/08/$25.00 ©2008 IEEE